1. Field of the Invention
The present invention relates to a ball screw, and more particularly, to a ball screw suitable for use as a feed mechanism for automatic doors, and so forth.
2. Description of the Prior Art
FIGS. 1 and 2 indicate one example of a ball screw of this type. Such as ball screw is disclosed in, for example, Examined Patent Publication No. 44-15206.
In the drawings, screw shaft 1 has 3 lines of threads, and nut 2 floats, i.e. is supported on screw shaft 1. Circular groove 2a is formed in the inner circumference of nut 2, and a number of balls 3 equal to the number of threads of screw shaft 1, namely 3, are arranged within the circular groove 2a. These balls 3 are each engaged with each of the three screw threads la of screw shaft 1.
The ball screw having the constitution described above is used, for example, in a feed mechanism of a door in an automatic door device. Namely, by attaching nut 2 to a door, providing screw shaft 1 on the stationary side, and rotating screw shaft 1 by applying torque to screw shaft 1 by a motor 1, nut 2 reciprocates along screw shaft to perform opening and closing operations of the door.
However, in an automatic door, operation should be performed at a relatively low speed, in comparison with the intermediate portion of the opening and closing stroke, at the portions before and after the intermediate portion do promote safety. This speed controlling operation was achieved by, for example, using a servo motor as the motor for torque generation, and controlling the angular speed of the servo motor.
However, in this constitution, in addition to requiring the use of a relatively expensive servo motor, it is also necessary to provide a control circuit for control of the speed of the motor. In addition to this resulting in increased costs, since shifts in the accelerating and slowing points as well as stopping position of the door occur due to drifting of the motor speed and so on, adjustment must be performed frequently.